Auto-focus control for digital cameras or video cameras employs a contrast method which sets, as an in-focus position, the position of a lens where the high-frequency component of a luminance signal provided by an image pick-up device such as a CCD sensor is at the maximum level.
Known contrast methods include a “climbing method” wherein a lens is moved in a direction in which the high-frequency component of a luminance signal (hereinafter referred to as an in-focus evaluation value (sharpness)) provided by an image pick-up device increases and the position of the lens where the in-focus evaluation value is at the maximum is set to an in-focus position, and a “full range scan method” wherein in-focus evaluation values are stored while a lens is driven throughout the distance-measuring range and the position of the lens corresponding to the maximum of the stored in-focus evaluation values is set to an in-focus position.
Description will now be made of the drive of a focus lens in these methods with reference to FIG. 16. The horizontal axis represents the position of the focus lens in an optical axis direction, while the vertical axis represents the in-focus evaluation value. A point A where the in-focus evaluation value is at the peak indicates the in-focus position.
An arrow S1 represents the drive track of the focus lens in the “climbing method.” In the “climbing method,” the focus lens starts scan from the infinity end, detects the slope of the curve representing the in-focus evaluation value, and is driven in a direction in which the in-focus evaluation value increases. An in-focus state is determined by reaching the peak of the in-focus evaluation value.
An arrow S2 represents the drive track of the focus lens in the “full range scan method.” In the “full range scan method,” the focus lens starts scan from the infinity end, is driven to the closest end, and then driven back to the position where the in-focus evaluation value is at the maximum.
Other known auto-focus methods often employed for film cameras include a method which utilizes the principles of triangulation and a method which utilizes phase difference detection.
The triangulation detection method involves applying pulse light toward a subject and detecting the light reflected therefrom by a light-receiving element placed at the distance of a predetermined baseline length from the light-applying device, thereby detecting the distance to the subject to drive a lens.
The phase difference detection method uses luminous flux passing through a plurality of different mirror areas of an image-taking lens for an imaging surface and has an optical system for secondary image formation of the luminous flux. The two images formed through the secondary image formation are detected by two line sensors, and the phase difference between the data of the two images is detected to determine the defocus state of the subject image to calculate an in-focus position. Then, predetermined lens drive is performed to achieve an in-focus state.
The abovementioned conventional contrast methods, however, have the problem of requiring a long time period for achieving focusing. Especially, the full range scan method detects an in-focus position (the maximum of the in-focus evaluation value) by scanning the full drive range from the infinity end to the closest end with the focus lens, so that the wide range must be scanned and a long time period is required to achieve focusing.
Even in the climbing method, it takes a long time period to achieve focusing when a standby position of the focus lens is located away from an in-focus position. In addition, since the in-focus evaluation value is not changed significantly at positions away from the in-focus position, it is difficult to know whether defocus is caused by front focus or rear focus, which may increase extremely the time taken to achieve focusing.
In the triangulation method and the phase difference detection method, if the focus lens stands by at the infinity end and an in-focus position is located near to the closest end, the focus lens must be driven over a long distance to the in-focus position, so that they have the problem of requiring a long time period to achieve focusing.